An oxide semiconductor made of BaTiO.sub.3 added with 0.1-0.3 at % of Y, Nd or the like, which has a large positive temperature coefficient, is known as "PTC thermistor".
The PTC thermistor, which can adjust its temperature range having large positive temperature coefficient by adding Sr, Pb or the like thereto, has been increasingly indispensable in various fields including temperature measurement, excess current prevention, motor start, a circuit element for demagnetization in a color television, and a constant temperature heater.
Such a thermistor comprises, as shown as an example in FIG. 5(a), a thermistor main body 11 of a thin cylindrical shape made by sintering an oxide, carbonate, nitrate or chloride of metals such as Ba, Ti, Nd or the like, first electrode layers 12a and 12b as Ni plated layers formed on upper and lower surfaces of thermistor main body, and second electrode layers 13a and 13b including silver as their main component and formed on the first electrode layers respectively.
Meanwhile, such a positive characteristic thermistor is usually used by applying a voltage across the second electrode layers 13a and 13b, in which case a so-called migration phenomenon takes place, that is, the silver contained in the second electrode layers is separated and moved toward a direction of the electric field. In particular, in the case where the outer periphery of the second electrode layers are formed to reach the outer periphery of the main body 1 of the positive characteristic thermistor, the silver element is separated and moved toward the direction of the electric field on the outer peripheral surface of the main body 1 of the positive characteristic thermistor, which eventually results in an undesirable short-circuit.
In order to solve this problem, there has been proposed a positive characteristic thermistor in which, as shown in FIG. 5(b), the outer diameter of the second electrode layers is smaller than that of the first electrode layers.
This structure, however, has had a problem that, since the contour of the second electrode layers is smaller than that of the first electrode layers, those parts of the first electrode layers not covered with the second electrode layers are exposed directly to atmosphere, which results in that those parts of the first electrode layers are liable to be oxidized and a contact resistance gradually increases.
Further, since the silver migration is a phenomenon in which the silver is separated and moved along the direction of the electric field, even when the second electrode layers alone are provided inside of the outer periphery as in the prior art, the silver is still diffused into the first electrode layers, though the quantity of the silver diffusion is very small. In this way, it has been impossible to completely prevent the above short-circuit problem, though it could be weakened.
Furthermore, since the electrode formation of the prior art positive characteristic thermistor is carried out by a known plating method, this method involves, during the Ni plating process of the electrodes, immersion of plating solution into the interior of the sintered body, thus resulting in that the characteristic of the sintered body is undesirably changed, e.g., its resistance value is decreased. This result may appear immediately after the electrode formation in the form of variations in the characteristic or may appear gradually with passage of time. As already mentioned earlier, thermistor applications require highly accurate control of its resistance value in all the fields including measurement, control and compensation of temperatures, gain adjustment, power measurement, overcurrent prevention, motor start, and demagnetization in a color television, that is, requires a range of R.+-..alpha. %. Accordingly, this problem of variations in the resistance value caused by the immersion of plating solution becomes serious.
Meanwhile, for the purpose of avoiding such a plating-solution immersion problem, there has been suggested to form an electrode of a metal having a low melting point such as aluminum by a metal spraying method.
However, this method also involves a problem that cracks occur in the thermistor body itself or electrodes themselves since the temperature abruptly changes during the electrode formation.
In this way, with the prior art structure having the second electrode layers formed to be smaller in the outer diameter than that of the first electrode layers, since those parts of the first electrode layers not covered with the second electrode layers are exposed directly to atmosphere, there has been a problem that those parts are liable to be oxidized and the contact resistance increases with time.
Further, in the prior art positive characteristic thermistor, the electrode formation is carried out by the plating method, which causes the immersion of plating solution into the sintered body during the Ni plating operation with the result of an undesirable change in the characteristic of the sintered body, e.g., its resistance value is decreased.
Furthermore, in case where such Ni plated layer is formed somewhat inside of the outer periphery of the thermistor main body, it is required to make a mask pattern of a resist or the like, immerse the main body into the Ni plating solution for Ni plating and then remove the mask pattern. In this case, the surface of the thermistor main body is liable to be polluted by metallic ions due to the contamination by the Ni plating solution and the stripping solution of the mask pattern, which might lead to the cause of variations in the resistance values or the cause of inducing the migration.
In this way, the prior art electrode formation methods using the Ni plating have had a problem that it is impossible to maintain favorable characteristics including a highly reliable resistance characteristic.
In view of the above circumstances, the present invention has been made to provide a thermistor having stable characteristics.